i am hopping on a speaker project using alpair 10.2 drivers. i wil go with either erich haartmann's bookshelf design or simple pencil box. although 10.2 can go down pretty well, i would like to augment the lower bass with subwoofer to take out some stress from the alpairs. i am looking for suitable subwoofer in a sealed box(for tighter bass). pls suggest one which will pair well with the alpairs. i could go for biamping with active xo

thats dipole right!? my requirement would be to place the sub somewhere in corner. moreover i am looking for tighter bass, so sealed box is required (correct me here). in case of H-frame, how much far they would need to be placed from back and side walls?

Yes the H frame is dipole. "Tighter bass" might mean different things to different people but to me it would mean clean, clear, true to the live event, and free from enclosure colorations. Dipole pressurizes the room differently than sealed (yes I have both although my sealed is a servo). Sealed can have more of that "bang in the chest' impact but not necessarily more "real life". H frame likes to be a bit away from the wall so tucked tight into a corner might not work (don't really know never tried that). You might also consider how far the corner placement is from the main speakers. More than 8 feet can be problematic, however adjustable phase control on your sub amp (not just those 0-180 switches) can help. Common wisdom is that subs crossed over under 80Hz are difficult to locate viz a viz the main speakers. Uncommon wisdom would say 50Hz is more like it. If it was my $400 I'd just build 2 of the H frames and enjoy the music. No matter what you do in the future I bet those H frames will be with you forever.

I'd be leary of mixing a dipole woofer with monopole top. I can also say that, having a pair of unused goldwood, i am not entralled with the build quality, its main asset is that it is big, And cheap. An H-frame will also have cavity resonance that means it is not as extended at the top as i like when integrating with a FR,

Yjru might be great, i have just not been inspired to try them. The Eminece 15" H-frames are built but haven't seen much use.

For a solid foundation on the bottom, i chose CSS SDX10, which i am running sealed.

I have my H frames crossover at 70Hz with 24dB slope so no cavity resonance issues really. However now that you mentioned you want a single sub with a corner placement, and since Dave likes the SDX10 (haven't heard them but I'd trust Dave's opinion) why not look at one of the kits that CSS sells. If you can stretch the budget a bit, CSS offers the Antimode digital subwoofer equalizer at a nice discount with their kits and I can tell you those things work the kind of magic you'll really appreciate integrating a single corner placed sub with your Alpair 10.2's. I heard the Antimode in a set up with twin tower subs of 6 peerless 12 inchers and all the boom disappeared and the real deal was there big as life.